Different types of cloud, for example, can help cool or warm the planet.

"We will be making the key observations that address this problem," said Dr Graeme Stephens, the Cloudsat principal investigator from Colorado State University, US.

The US space agency (Nasa) satellites were launched from the Vandenberg Air Force Base in California at 1002 GMT, after a week of delays due to technical problems and unfavourable wind conditions.

They have been put in a 705km (438 miles) circular, sun-synchronous polar orbit, where they will fly in formation just 15 seconds apart.
The spacecraft are part of an Earth-observation constellation Nasa calls the "A-Train".

A tiny, tiny fraction of the water on our planet is in clouds and yet that tiny, tiny fraction is what provides the fresh water on which humans depend

Graeme Stephens, Colorado State University

It sends short pulses of microwave energy down into the atmosphere, and by recording the way these pulses are scattered back to the satellite obtains a picture of the structure and water content of clouds.

"The strength of the return from the radar is actually directly related to the amount of water that's in clouds. Effectively, it allows us to weigh the clouds," explained Dr Stephens.

"The time delay of the pulses means we can look at different levels and that gives us the profile of clouds."

It uses lidar, which fires pulses of shorter wave energy - in the infrared and visible part of the spectrum - down into the atmosphere to obtain a different, but complementary, set of data from Cloudsat's.

In particular, Calipso is concerned with aerosols. These very fine particles are thrown up into the atmosphere by natural processes, such as volcanic eruptions, dust and sand storms, and even sea spray.

Aerosols take very complex forms, and contain a range of chemical compounds. They can be solid or liquid, or even solid material inside a drop of liquid.

And their longevity is highly varied, with some aerosols lasting a few days and others hanging in the air for months.

"The bottom line here is that to measure aerosols is very difficult; you need a variety of instruments," explains Dr Charles Trepte, Calipso project scientist from the US space agency's Langley Research Center.

"Nasa and other agencies have been making measurements of aerosols from space for many years, but the problem is that they haven't been able to measure all the properties; and one thing they are missing is the vertical distribution of aerosols in thin clouds."

Different ways

Aerosols have a fundamental relationship with clouds by providing the nuclei on which cloud droplets can form.

Clouds that form in clean air are made up of droplets that tend to get bigger because they form on fewer nuclei; and these clouds tend to rain more, too.

Clouds that develop in dirty air form many more, but smaller, droplets. These clouds also look brighter.

Calipso looks at how aerosols interact with clouds

By picking apart these details, Calipso will help scientists understand the direct and indirect effects of aerosols on climate.

"Directly, they can scatter sunlight back to space and have a cooling effect just by reducing the amount of solar radiation reaching the Earth system," said Dr Trepte.

"They can also absorb solar radiation and warm the atmosphere, possibly alter circulations, change the thermal stability of the atmosphere and perhaps inhibit convection.

"And then they have the indirect effects of changing cloud properties, allowing them to last longer, changing the amount of precipitation - either increase it or decrease it - and perhaps even dim or brighten clouds so that they have better or worse reflecting properties."

The Cloudsat and Calipso missions have a number of objectives:

Together, the satellites will provide the first statistics on the vertical structure of clouds. Scientists will be able to see clouds from their tops to their bottoms - like getting a CT scan of clouds from space

Cloudsat and Calipso will give researchers the first indirect but validated estimate of how much clouds and aerosols contribute to the vertical distribution of atmospheric warming

Cloudsat will provide the first global estimates of the percentage of Earth's clouds that produce rain

Cloudsat will afford scientists the first vertically sliced picture of how much water and ice are in Earth's clouds

Cloudsat will provide the first ability to detect snowfall from space

Cloudsat will offer the first estimates of how efficiently the atmosphere produces rain from condensates

Calipso will provide the first statistics on the global vertical distribution of aerosols and aerosol types

Calipso will reveal for the first time how often "sub-visible" cirrus clouds - very thin clouds invisible to the naked eye - occur, and whether they change with the seasons

The new understanding obtained through the spacecraft will be fed into computer models, to improve their predictions. This should lead not just to better weather and air quality forecasts, but to reduced uncertainties in our expectations of future climate change.

"A tiny, tiny fraction of the water on our planet is in clouds and yet that tiny, tiny fraction is what provides the fresh water on which humans depend," Dr Stephens said.

"Clouds replenish our fresh water resources and yet we can't really tell you today how clouds will change under the pressures of global climate change."

Cloudsat and Calipso join a fleet of other satellites - known as the A-Train - which are aiming to give a rounded picture of Earth's atmospheric and water systems.

The spacecraft circle the planet on a path that takes them over broadly the same observation point in quick succession.

The platforms carry different instrumentation to address specific questions.

1. Oco will launch in 2008 and head the train. It will measure the concentration of carbon dioxide (CO2) in the atmosphere

2. Aqua will lag Oco by 15 minutes. It is collecting information about the Earth's water cycle - water in the oceans, the air and on the land

3. Cloudsat will allow for the most detailed study of clouds to date. It should better characterise their role in regulating the climate

4. Calipso views clouds just moments after Cloudsat has looked at them. Its primary interest is the way aerosols interact with clouds

5. Parasol is a French satellite that can distinguish natural from human-produced aerosols. It makes polarised light measurements

6. Aura also has a big European investment. It looks at atmospheric chemistry, and is producing remarkable global pollution maps